In order to improve the properties of a metal material such as steel, the metal material is often subjected to a heat treatment such as quenching, tempering, annealing and normalizing. In the above heat treatments, quenching is a treatment according to which a heated metal material is immersed in a coolant to be transformed into a predetermined hardened structure. As a result, the quenched object becomes considerably hard. For instance, when heated steel in an austenitic phase is immersed in a coolant and cooled at an upper critical cooling rate or higher, the steel can be transformed into a hardened structure such as martensite.
As a coolant, an oil- or water-soluble quenchant is typically used. Now, quenching of steel is explained. When heated steel is put in a heat-treatment oil (i.e., coolant), the steel is not cooled at a constant cooling rate but usually cooled through the following three stages (1) to (3). Specifically, the steel is cooled through (1) a first stage where the steel is surrounded by the steam of the heat-treatment oil (i.e., a vapor blanket stage), (2) a second stage where a vapor blanket is ruptured and boiling occurs (i.e., a boiling stage) and (3) a third stage where the temperature of the steel falls below the boiling point of the heat-treatment oil and the heat of the steel is absorbed by convection (i.e., a convection stage). The cooling rate becomes the highest at the second stage (boiling stage) of the above three stages.
Generally, when the heat-treatment oil is used, the cooling rate is rapidly increased especially at the boiling stage. Therefore, when a surface of the object to be treated experiences a transitional phase where the vapor blanket stage and the boiling stage are mixed, the surface of the object to be treated is subjected to an extremely large temperature difference. Such a temperature difference results in a difference in thermal contraction and time lag of transformation and thus in generation of thermal stress and transformation stress, which increases quenching distortion.
Accordingly, for a heat treatment, especially quenching, of a metal, it is important to select a heat-treatment oil suitable for heat-treatment conditions. When an unsuitable oil is selected, the metal is likely to be insufficiently hardened and have a severe distortion.
Heat-treatment oils are categorized into Classes 1 to 3 according to JIS K 2242 and oils of Class 1, No. 1 and No. 2 and Class 2, No. 1 and No. 2 are used for quenching. Regarding the above oils, JIS K 2242 defines, as an index of cooling performance, a cooling time (sec) required for reduction from 800 degrees C. to 400 degrees C. according to the JIS cooling curve. Specifically, the cooling time of the oils of Class 1, No. 2 is defined as 4.0 seconds or less, Class 2, No. 1 is 5.0 second or less, and Class 2, No. 2 is defined as 6.0 seconds or less. When the cooling time is shorter, it means that the cooling performance is higher and thus the hardness of the heat-treated object is further enhanced. Generally, hardness and quenching distortion are in a trade-off relationship, which means that a higher hardness results in a larger quenching distortion.
Industrially, an H-value is also frequently used as an index of the cooling performance of a quenching oil. The H-value can be calculated from a cooling time required for reduction from 800 degrees C. to 300 degrees C. according to the JIS cooling curve. In order to achieve desired hardness and quenching distortion, a user selects a quenching oil based on the above indexes. For instance, for quenching an automobile gear or the like, for which distortion may cause a problem, an oil of JIS Class 2, No. 1 is frequently used. This is because an oil of JIS Class 1 increases distortion and excessively enhances the hardnesses of some components. In contrast, an oil of Class 2, No. 2 reduces distortion but cannot provide a sufficient hardness.
Components of automobile transmission, reducer and the like are usually mass-produced and thus subjected to so-called mass-quenching, according to which a large number of objects to be treated are piled up on one tray and subjected to quenching at the same time. The thus-hardened components vary in hardness and distortion depending on where the components are arranged in the pile. For instance, while the components arranged near the bottom of the pile have a high hardness, the components arranged near the top of the pile have a low hardness.
In order to reduce such an unevenness accompanying the mass-quenching, it has been considered that specific equipment such as a vibrator and an injector is added according to Patent Literature 1. However, addition of such equipment to a typical device is costly and it is difficult to reconstruct the device to be added with some types of equipment. Accordingly, in order to avoid such equipment investment, it has been desired to develop a technique for reducing the unevenness only based on the properties of a quenching oil. For instance, it has also been considered that an object to be treated is cooled below the characteristic temperature of a quenching oil using gas prior to oil-quenching in order to reduce the influence of the characteristic time (sec) of the quenching oil (Patent Literature 2) and that an object to be treated is temporarily taken out of a quenching oil to be soaked when heated to a martensite-transformation-start temperature in order to eliminate a temperature difference in the object to be treated resulting from uneven cooling (Patent Literature 3). However, both methods are more costly and more time-consuming than a simple quenching. Further, these methods, which are intended to reduce distortion, cannot reduce distortion unevenness.
Patent Literature 4 discloses a heat-treatment oil composition that is less likely to unevenly cool a metal material to be quenched and reliably provides hardness to the quenched material while reducing quenching distortion, the heat-treatment oil composition containing a mixed base oil of a low-viscosity base oil with a kinematic viscosity at 40 degrees C. of 5 to 60 mm2/s in an amount of 50 to 95 weight % and a high-viscosity base oil with a kinematic viscosity at 40 degrees C. of 300 mm2/s or more in an amount of 50 to 5 weight %. However, Patent Literature 5 teaches that the composition prepared based on the above composition range provides excessive hardness when used for an automobile gear or the like.
Patent Literature 5 discloses a quenching oil composition capable of reducing unevenness of cooling performance during mass-quenching. Especially, Patent Literature 5 discloses a quenching oil composition that exhibits a cooling performance similar to that of an oil of JIS Class 2, No. 1 (i.e., an oil used for quenching components of automobile transmission or reducer where distortion causes a problem) and is capable of reducing unevenness of the cooling performance during mass-quenching. Specifically, the disclosed composition is a heat-treatment oil composition containing a mixed base oil of a low-boiling-point base oil with a 5%-distillation temperature in a range of 300 to 400 degrees C. in an amount of 5 mass % to 50 mass % and a high-boiling-point base oil with a 5%-distillation temperature of 500 degrees C. or higher in an amount more than 50 mass % but not more than 95%. However, further examination has revealed that the composition according to the invention of Patent Literature 5 cannot always improve distortion unevenness depending on the shape of a gear.